With the need for smaller size, lower weight and higher functionality in portable electronic devices such as smart phones, digital cameras and handheld gaming devices, active progress is being made recently in the development of high-performance batteries and demand for secondary batteries, which can be repeatedly used by charging, is growing rapidly.
Lithium ion secondary batteries in particular, because of their high energy density and high voltage, and also because they lack a memory effect during charging and discharging, are the secondary batteries currently undergoing the most vigorous advances in development.
In addition, as part of recent efforts to tackle environmental problems, active progress is also being made in the development of electrical vehicles, and higher performance has come to be desired of the secondary batteries that serve as the power source for such vehicles.
Lithium ion secondary batteries have a structure with a container in which are housed a positive electrode and a negative electrode capable of intercalating and deintercalating lithium and a separator interposed between the electrodes, and which is filled with a electrolyte (in the case of lithium ion polymer secondary batteries, a gel-like or completely solid electrolyte instead of a liquid electrolyte).
The positive and negative electrodes are generally formed by placing a composition which includes an active material capable of intercalating and deintercalating lithium, a conductive material consisting primarily of a carbon, material and a polymer binder as a layer on a current collector made of copper foil, aluminum foil or the like. The binder is used to bond the active material with the conductive material, and also to bond these with the metal foil. Fluoropolymers such as N-methylpyrrolidone (NMP)-soluble polyvinylidene fluoride (PVdF), aqueous dispersions of olefin polymers and the like are commercially available as such binders.
As noted above, lithium ion secondary batteries also show promise in use as a power source for electric vehicles and the like, and so it is desired that such batteries have a longer life and better safety than has hitherto been achieved.
However, the bonding strength of the above binders to the current collector is less than adequate. During production operations such as electrode plate cutting steps and winding steps, some of the active material and conductive material peels from the current collector and falls off, giving rise to micro-shorting and variability in the battery capacity.
In addition, with long-term use, due to swelling of the binder on account of the liquid electrolyte or to changes in the volume of the electrode mixture associated with volume changes resulting from lithium intercalation and deintercalation by the active material, the contact resistance between the electrode mixture and the current collector increases or some of the active material or conductive material peels from the current collector and falls off, leading to a deterioration in the battery capacity and leading also to problems from the standpoint of safety.
In particular, advances have been made recently in the development of active materials which, in positive electrode systems, are solid solution systems and, in negative electrode systems, are alloy systems of silicon or the like. These active materials have a larger charge/discharge capacity than pre-existing active materials, and thus experience a larger change in volume with charging and discharging. As a result, the peeling of such electrode mixtures from the current collector is a problem in urgent need of a solution.
Techniques that involve inserting an electrically conductive bonding layer between the current collector and the electrode mixture have been developed as attempts to solve the above problems.
For example, Patent Document 1 discloses the art of disposing, as a bonding layer between the current collector and the electrode mixture, an electrically conductive layer in which carbon serves as a conductive filler. This publication indicates that, by using a composite current collector having a conductive bonding layer (also referred to below simply as a “composite current collector”), the contact resistance between the current collector and the electrode mixture can be decreased, loss of capacity during high-speed discharge can be suppressed, and deterioration of the battery can be minimized. Similar art is disclosed also in Patent Documents 2 and 3.
In these examples, carbon particles are used as the conductive filler, but because carbon particles do not have a bonding action with respect to the current collector, a bonding layer is created using a polymer that serves as a matrix. Of course, the bonding strength rises as the polymer content becomes larger. However, as the polymer content increases, contact between the carbon particles decreases, and so the resistance of the bonding layer rises abruptly. As a result, the resistance of the battery as a whole rises.
To solve such problems, examples have been reported in which a fibrous carbon such as carbon nanotubes (abbreviated below as “CNTs”) is used as the conductive filler.
For example, Patent Document 4 reports the use of multi-walled carbon nanotubes (abbreviated below as “MWCNTs”) as the CNTs to form a conductive bonding layer on an aluminum foil, thereby making it possible to increase the cycle life of lithium ion secondary batteries. However, in Patent Document 4, because the dispersant used when forming the MWCNT-containing conductive bonding layer has a low dispersibility, it has been necessary to carry out spray coating a plurality of times in order to obtain a layer of sufficient film thickness.
In order to form a film by a CNT coating process, it is generally necessary to uniformly disperse CNTs in a solvent. There are techniques which involve surface modifying CNTs by a chemical process, and techniques which involve the concomitant use of a dispersant such as a polymer. Of these, techniques involving the concomitant use of a dispersant do not worsen the excellent electrical properties of CNTs, and so may be regarded as a preferred method in cases where the CNTs are used as an electrically conductive filler.
However, dispersants capable of dispersing CNTs to a high concentration have a low adhesion to the current collectors used in secondary batteries. To obtain a binder layer having excellent electrical conductivity, it is necessary to add a polymer or the like having the ability to adhere to the current collector. A problem in such cases is that the CNT concentration within the conductive binder layer decreases, as a result of which the electrical conductivity declines.